Exhaust gases of internal combustion engines can typically be cleaned catalytically. The exhaust gas hereby passes over at least one catalyst, which converts one or several pollutant components of the exhaust gas. Different types of catalysts are known. Oxidation catalysts promote the oxidation of un-combusted carbohydrates (HC) and carbon monoxide (CO), whereas reduction catalysts support reduction of nitrogen oxides (NOx) in the exhaust gas. The aforementioned components (HC, CO, NOx) can also be simultaneously converted catalytically by using 3-way catalysts. 
In addition, storage catalysts, such as NOx storage catalysts, are also known. These are used to clean exhaust gases of internal combustion engines which are operated at least temporarily in lean operating mode, i.e., with an oxygen-rich exhaust gas with λ>1, to optimize fuel consumption, producing large quantities of nitric oxides NOx. NOx cannot be entirely converted by an oxidizing catalytic conversion of unburnt hydrocarbons HC and carbon monoxide CO to environmentally neutral nitrogen. This situation can be remedied by locating the aforementioned NOx storage catalysts in the exhaust channels of internal combustion engines, which during the lean operating phases store NOx as nitrate. The NOx storage catalyst must be regenerated from time to time by switching the internal combustion engine into a rich or sub-stoichiometric operating mode (λ≦1).
The aforementioned catalysts age when operating at high temperatures, which reduces the peak conversion rate compared to an undamaged catalyst. To reduce aging of the catalyst, the maximum allowable temperature in the exhaust gas system is monitored and is limited by adjusting operating parameters of the engine, preferably the lambda value of the combustion process.
On the other hand, it is necessary to heat the catalyst, i.e., to introduce energy into the exhaust gas system, so as to reach the optimal temperature window of the catalyst and to desulfurize the NOx catalysts, since these tend to be poisoned by sulfur contained in the fuel. Heating the catalyst removes the sulfur stored in the NOx catalysts. A minimum temperature of approximately 600° C. is required to desorb the sulfur which is stored in the form of sulfate.
Heating the catalysts poses problems in particular when using pre-catalysts and main catalysts, because the pre-catalysts may be thermally overloaded when the main catalyst is brought to the desired temperature.
Energy can be intentionally introduced into the exhaust gas cleaning system, preferably for desulfurizing NOx storage catalysts, by heating the catalyst, in particular a main catalyst, by simultaneously exposing the catalyst to lean and rich exhaust gas. For example, to achieve a desired lambda value of the exhaust gas before the main catalyst of 1.0, the exhaust gas is shifted in one of the two exhaust gas paths by a predetermined amount toward “rich”, and likewise in the other path into the opposite direction. With this approach, the exhaust gas mix before the catalyst advantageously simultaneously contains high oxygen and pollutant concentrations. As a result, a large amount of chemically bound energy is converted in the catalyst. Because the energy required for heating the catalyst is converted to heat only in the catalyst, thermal losses on the path through the non-adiabatic exhaust gas system are eliminated. Moreover, any existing pre-catalysts are not thermally overloaded, which significantly extends their service life reliability.
This approach may introduce an increased risk of overloading the main catalyst, because at least a local thermal overload may not be preventable due to strong catalytic activity by a high conversion of chemical energy in the main catalyst. Moreover, a catalyst can be heated by a lambda split only, if the catalyst is at least partially active, i.e., is heated, because the catalyst must be thermally active for introducing the chemical energy.